backscattering

an ideal or imaginary body that is absolutely black when cold, but
is a perfect absorber of radiation and at the same time a perfect radiator.
Black body temperatures are used to work out the theoretical laws of
radiation and to calculate the temperature of the sun.

diurnal motion

eccentricity

the degree of flattening of an ellipse and how much it deviates from
a circular shape. With reference to the orbit of a planet, the distance
between the center of its orbit and the center of the
primary about which it revolves.

escape velocity

the speed that any object must acquire in order to escape from a planet's
gravitation

fluorescence

the emission of electromagnetic radiation as the result of exposure
to some other radiation, which emission ceases when the stimulus that
produces it ceases to act on the emitting substance. Thus light passing
through a gas, may change the energy state of the atoms and cause them
to emit light of a different wavelength.

a great circle of the celestial sphere passing through the celestial
poles and a celestial body or the vernal equinox. An hour circle moves
with the body as the celestial sphere rotates, unlike the celestial
meridian of a point that remains fixed.

interference fringes

the alternate light and dark bands that are seen when two beams of
homogenous light enter into interference with each other, i.e., when
they overlap and illuminate an identical area

interferometer

any instrument designed for producing and studying interference of
two or more trains of waves or beams of electromagnetic radiation of
the same range of wavelengths. Usually denotes an optical interferometer
that separates a beam of light into two or more parts that travel in
different paths and are then combined together to produce interference
fringes.

the point in a planet's orbit in which it is closest to Earth. When
viewing a planet it will appear on the opposite side of the sky as the
sun. Two bodies are in opposition when the difference in celestial longitude
is 180 degrees.

perihelion

quadrature

two bodies are in quadrature when their difference in celestial longitude
is 90 degrees. This is the position of a superior planet when its elongation
is 90 degrees, east or west, depending on the direction of the planet
from the Sun

retrograde motion

backward (westward) motion of the planet among the stars, resulting
from the fact that it is viewed from the moving Earth. It is the motion
that a planet has when its right ascension is decreasing.

Each year, when Earth passes a superior planet, such as Mars, the
planet appears to move backward for a short time. It is the same effect
you see when two trains are running in the same direction on side
by side tracks. The slower train appears to be moving backward.

a celestial coordinate; it is the angular distance measured from the
vernal equinox eastward along the celestial equator to the hour circle of a given star or other celestial body. It can be
defined also as the arc of the celestial equator or the angle at the
celestial pole between the hour circle of the vernal equinox and the
hour circle of the body.

ultraviolet radiation

vernal equinox

the position on the celestial sphere occupied by the sun on or about
March 21 when its path on the ecliptic crosses the celestial equator
going from south to north. This point is also called the March equinox
and the First Point of Aries.